# The organic prefix "but-" means that the organic compound has how many carbon atoms?

You mean $\text{butyl}$, and this is a ${C}_{4}$ group.
$\text{n-Butane}$ $=$ ${H}_{3} C - C {H}_{2} C {H}_{2} - C {H}_{3}$.
$\text{isobutane}$ $=$ ${H}_{3} C - C H \left(C {H}_{3}\right) C {H}_{3}$ or $H C {\left(C {H}_{3}\right)}_{3}$.
Of course, when the $\text{butyl}$ group is substituted somewhere we get a ${C}_{4} {H}_{9}$ residue.